When a Session Initiation Protocol (SIP) user agent client (UAC) establishes a communication session, the UAC follows a series of message exchanges with servers in the network. First, the startup procedure involves a SIP registration with the servers. To subscribe to event packages during the communication session, the UAC sends a separate subscription message to the appropriate server. This traditional technique results in numerous messages between the UACs and the servers. Furthermore, each message may be periodically refreshed, which causes additional traffic.
The multitude of messaging is particularly ineffective during an avalanche restart that causes UACs to simultaneously re-register with the servers and re-subscribe to the event packages. The additional messages transmitted between the UACs and the servers cause the system to take longer to return to service. Furthermore, the multitude of messages also presents problems on wireless links and other interfaces where bandwidth is at a premium.